The present invention relates generally to the field of audio broadcast and recording instrumentation and, more particularly, to an improved loudness meter and method having a dual response characteristic.
Various meters and metering standards are in use throughout the world to indicate audio program loudness levels. The most well-known standard meters include the VU meter described in ANSI Standard C 16.5R of 1961 and the Peak Power Meter (PPM) described in the Standard IEC 268-10.
The VU meter was developed to define program levels from different sources under dynamic conditions for the use in telecommunications broadcast interface. The VU meter is basically a galvanometer with specific meter ballistics, calibrated in power. The specific meter ballistics require a ninety-nine percent deflection with one percent overshoot upon the application of a sinusoidal voltage of reference amplitude and a rise time of 300 millicycles. It also requires that the decay time fall with the same characteristics. Only meters of this dynamic behavior can be termed VU meters. Because of the substantial integration of program material over the response time of the VU meter the same does not give an accurate representation of short term or peak signal amplitudes. Broadcast engineers have generally gained experience with such meters and are often able to correctly compensate level adjustments so as to equalize the loudness level between peak type sources and persistent sources. Such adjustments are not consistent and are not based upon reproducable experience based on meter readings. Thus, those who work with the VU meter relate its usefulness in setting program levels by adding certain compensations which are generally known in the trade. Such compensations including the riding of dialogue to a level 3 to 5 dB below music and placing drums as much as -10 dB below music.
In multi-channel music recording a VU meter rides on each channel and is dedicated to some very small cluster of instruments or a single instrument. The recording engineer acquires a knowledge that given instruments must ride so many db below other instruments or collections of other instruments in order to produce an equal loudness experience to a listener.
In these operations, it is not possible for inexperienced persons to make properly balanced recordings or to control with equal loudness alternating sources of audio input in radio and television broadcasting circuits and even the experts rely on personal expertise.
Thus, out current method of monitoring program levels is found to be inadequate. The standard VU Meter as used in the broadcast industry today does not give a true indication of perceived audio level. A simple experiment on any VU Meter will verify this. Connect a tone generator to an amplifier bridged by a VU Meter. Establish a "0" level and discern the loudness. Now short the generator in rapid session (beep, beep, beep, etc.). Notice that the apparent loudness remains the same but the meter indicator drops several dB. The meter is incapable of showing an accurate reading.
Relating this to the complexities of broadcast audio a typical problem that the VU Meter cannot deal with can be explained in this fashion. Anything musical such as a bass, guitar and piano in composition allows the VU Meter to gain some footing and show a fairly close average to "0" VU whereas a male voice which is very staccato at an average "0" VU reading would in reality be twice as loud.
The erroneous readings of perceived audio in the VU Meter are not just ballistic problems as the VU Meter has a full wave rectifier that converts AC to DC which activates the meter. For example, program sources that are highly asymmetrical such as a solo male voice act upon the rectifier differently than program material that appears more sinuoidal such as music.
The two standard VU Meters used in stereo broadcast create another problem. Consider the following situation and outcome. Generally, two VU Meters appear on the stereo control console. The announcer speaks through both channels to appear mono in the center. His voice level is adjusted to relative "0" on both VU Meters. A transition is made to a stereophonic recording with piano on the left showing a "0" VU reading, and a guitar on the right also showing a "0" VU reading. This typical setup results in the announcer sounding twice as loud because he has twice the accoustical efficiency resulting from his being electrically connected to two channels.
The summation of both left and right on a Total Meter would improve the noted distribution of power overall in the above situation except one is still faced with the ballistics problem inherent in the standard VU Meter.
Broadcast engineers have noticed for years that voice announcements and commercials work audio processing equipment far harder than do recordings as the factors in the above analysis are directing the outcome. Consequently, the broadcaster has had to work with a deeper range with resultant degradation quality in order to hold a more uniform output.
The recording industry is sometimes trapped in this same situation. When instruments, especially those of the low frequency range such as a bass, are mixed on both left and right channels as to appear in the center the resultant mono product contains too much bass. Sometimes this maybe done intentionally and other times it is the outcome of mixing while monitoring both left and right channels. This is commonly known as center channel buildup.
The development of transistor technology placed greater emphasis on the nature of peak amplitudes of program material and has reasserted the need for an alternative indicator to the VU meter. The so-called PPM meter, long popular in Europe, is a meter ballistic with a rise time of ten milliseconds, i.e., thirty times faster, than the ballistics of the VU meter. This allows an indication of peak amplitudes normally ignored by the VU meter. Fall times of the PPM are considerably slower than that of the VU meter, however, and vary from 1.7 seconds to 2.8 seconds depending upon the standard employed. If a studio or facility elects to control its audio level with PPM meters is faces similar outcome as that of the VU meter, i.e., there must exist experienced persons who know the relative loudness of different sources from experience and relate this level to the visual indications of the meter standard. Level adjustement compensations are not convertible for people to communicate the balanced condition when proceeding from source to source.
Although numerous other meters and other metering systems have been designed and marketed in an effort to correct the foregoing limitations and disadvantages, such devices again vary considerably in their ballistics, scale design and the type of readout and have not served to unify the art of loudness level measurement or adjustment. Of course, the simultaneous employment of a VU meter with a PPM meter might be considered, but the outcome requires the operator to keep track of two sets of compensations in order to understand the effect on loudness. An example of a typical combination of peak and average power reading meters using an LED display is set forth in U.S. Pat. No. 4,166,245 to Roberts issued Aug. 28, 1979. However, the relationship and character of the loudness signals simultaneously displayed leaves unresolved the problem of the user combining known or unknown compensations for program material so as to achieve for all program materials an equal loudness impression.
As a general indication of the background of this field, the reference made to the article entitled "A New Standard Volume Indicator and Reference Level" by Chinn, et al. a copy of which is lodged together with this application and is available from the Patent Office. As there pointed out, the VU meter characteristics are rather arbitrary and are selected as a compromise so as to provide a single reading instrument which would have wide applicability. Also the instrument should be compatable with telephone as well as broadcast circuits. The article indicates that it would be desirable to have both an RMS and peak reading instruments, but that at that time it was not possible to achieve the same within the constraints of available instrumentation possibilities given. Also the article points our that the measurement of actual program material has to be considered as distinctly different from the mere statement of the possibility of measuring the characterization of a sine wave input. Thus, the Chinn article analyzes various program materials and shows considerable difference in loudness between them, the effects of phase cancellations, and other variables. It further points out that it cannot be said that a meter is precisely a peak reading meter, a RMS meter, a square law meter, or an averaging meter since they believed that all metering systems are of some intermediate character. The Chinn article states that probably the most important thing not usually measured directly is the time over which the complex loudness wave is integrated by the meter. This characteristic is not to be found directly measured even with some sophisticated electronics. There is, therefore, a need for an improved loudness meter and method which will overcome the above limitations and disadvantages.